Network Working Group S. Farrell
Request for Comments: 5327 Trinity College Dublin
Category: Experimental M. Ramadas
ISTRAC, ISRO
S. Burleigh
NASA/Jet Propulsion Laboratory
September 2008
Licklider Transmission Protocol - Security Extensions
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
IESG Note
This RFC is not a candidate for any level of Internet Standard. It
represents the consensus of the Delay Tolerant Networking (DTN)
Research Group of the Internet Research Task Force (IRTF). It may be
considered for standardization by the IETF in the future, but the
IETF disclaims any knowledge of the fitness of this RFC for any
purpose and in particular notes that the decision to publish is not
based on IETF review for such things as security, congestion control,
or inappropriate interaction with deployed protocols. See RFC 3932
for more information.
Abstract
The Licklider Transmission Protocol (LTP) is intended to serve as a
reliable convergence layer over single-hop deep-space radio frequency
(RF) links. LTP does Automatic Repeat reQuest (ARQ) of data
transmissions by soliciting selective-acknowledgment reception
reports. It is stateful and has no negotiation or handshakes. This
document describes security extensions to LTP, and is part of a
series of related documents describing LTP.
This document is a product of the Delay Tolerant Networking Research
Group and has been reviewed by that group. No objections to its
publication as an RFC were raised.
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RFC 5327 LTP - Extensions September 2008
Table of Contents
1. Introduction ....................................................22. Security Extensions .............................................22.1. LTP Authentication .........................................32.2. A Cookie Mechanism .........................................63. Security Considerations .........................................74. IANA Considerations .............................................75. Acknowledgments .................................................86. References ......................................................86.1. Normative References .......................................86.2. Informative References .....................................91. Introduction
This document describes extensions to the base LTP protocol
[LTPSPEC]. The background to LTP is described in the "motivation"
document [LTPMOTIVE]. All the extensions defined in this document
provide additional security features for LTP.
LTP is designed to provide retransmission-based reliability over
links characterized by extremely long message round-trip times (RTTs)
and/or frequent interruptions in connectivity. Since communication
across interplanetary space is the most prominent example of this
sort of environment, LTP is principally aimed at supporting "long-
haul" reliable transmission in interplanetary space, but has
applications in other environments as well.
This document describes security extensions to LTP, and is part of a
series of related documents describing LTP. Other documents in this
series cover the motivation for LTP and the main protocol
specification. We recommend reading all the documents in the series
before writing code based on this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [B97].
2. Security Extensions
The syntactical layout of the extensions are defined in Section 3.1.4
of the base protocol specification [LTPSPEC].
Implementers should note that the LTP extension mechanism allows for
multiple occurrences of any extension tag, in both (or either) the
header or trailer. For example, the LTP authentication mechanism
defined below requires both header and trailer extensions, which both
use the same tag.
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RFC 5327 LTP - Extensions September 2008
This document defines new security extensions for LTP but does not
address key management since key management in Delay-Tolerant
Networking (DTN) remains an open research question.
If LTP were deployed layered on top of UDP, it might be possible to
use IPsec or other existing security mechanisms. However, in general
DTN, IPsec's key exchange (IKE) cannot work (e.g., where link delays
are measured in minutes).
2.1. LTP Authentication
The LTP authentication mechanism provides cryptographic
authentication of the segment.
Implementations MAY support this extension field. If they do not
support this header, then they MUST ignore it.
The LTP authentication extension field has the extension tag value
0x00.
LTP authentication requires three new fields, the first two of which
are carried as the value of the Extensions field of the LTP segment
header, and the third of which is carried in the segment trailer.
The fields that are carried in the header extensions field are
catenated together to form the extension value (with the leftmost
octet representing the ciphersuite and the remaining octets the
KeyID). The KeyID field is optional, and is determined to be absent
if the extension value consists of a single octet.
Ciphersuite: an 8-bit integer value with values defined below.
KeyID: An optional key identifier, the interpretation of which is
out of scope for this specification (that is, implementers MUST
treat these KeyID fields as raw octets, even if they contained an
ASN.1 DER encoding of an X.509 IssuerSerial construct [PKIXPROF],
for example).
The LTP-auth header extension MUST be present in the first segment
from any LTP session that uses LTP authentication, but MAY be omitted
from subsequent segments in that session. To guard against
additional problems arising from lost segments, implementations
SHOULD, where bandwidth allows, include these fields in a number of
segments in the LTP session. If the first segment (or any part
thereof) is retransmitted, then the LTP-auth header extension MUST be
included in the retransmission.
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The field carried as a trailer extension is the AuthVal field. It
contains the authentication value, which is either a message
authentication code (MAC) or a digital signature. This is itself a
structured field whose length and formatting depend on the
ciphersuite.
If for some reason the sender includes two instances of LTP-auth
headers, then there is a potential problem for the receiver in that
presumably at least one of the AuthVal fields will not verify. There
are very few situations where it would make sense to include more
than one LTP-auth extension in a single segment, since LTP is a peer-
to-peer protocol. If however, keys are being upgraded, then the
sender might protect the segment with both the new and old keys. In
such cases, the receiver MUST search and can consider the LTP
authentication valid so long as one AuthVal is correct.
For all ciphersuites, the input to the calculation is the entire
encoded segment including the AuthVal extension tag and length, but
not of course, including the AuthVal value.
We define three ciphersuites in this specification. Our approach is
to follow the precedent set by TLS [TLS], and to "hardcode" all
algorithm options in a single ciphersuite number. This means that
there are 256 potential ciphersuites supported by this version of
LTP-auth. Since this is a limited space, IANA has established a
registry for LTP Ciphersuites as described in the IANA Considerations
section below. Current ciphersuite assignments are:
Ciphersuite Value
----------- -----
HMAC-SHA1-80 0
RSA-SHA256 1
Unassigned 2-127
Reserved 128-191
Private/Experimental Use 192-254
NULL 255
1. HMAC-SHA1-80 Ciphersuite
The HMAC-SHA1-80 ciphersuite involves generating a MAC over the
LTP segment and appending the resulting AuthVal field to the end
of the segment. There is only one MACing algorithm defined for
this, which is HMAC-SHA1-80 [HMAC]. The AuthVal field in this
case contains just the output of the HMAC-SHA1-80 algorithm, which
is a fixed-width field (10 octets).
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RFC 5327 LTP - Extensions September 2008
2. RSA-SHA256 Ciphersuite
The RSA-SHA256 ciphersuite involves generating a digital signature
of the LTP segment and appending the resulting AuthVal field to
the end of the segment. There is only one signature algorithm
currently defined for this, which is RSA with SHA256 as defined in
[RSA], Section 8.2. The AuthVal field in this case is simply the
signature value, where the signature value occupies the minimum
number of octets, e.g., 128 octets for a 1024-bit signature).
3. NULL Ciphersuite
The NULL ciphersuite is basically the same as the HMAC-SHA1-80
ciphersuite, but with a hardcoded key. This ciphersuite
effectively provides only a strong checksum without
authentication, and thus is subject to active attacks and is the
equivalent of providing a Cyclic Redundancy Check (CRC).
The hardcoded key to be used with this ciphersuite is the
following:
HMAC_KEY : c37b7e64 92584340
: bed12207 80894115
: 5068f738
(The above is the test vector from RFC 3537 [WRAP].)
In each case, the bytes that are input to the cryptographic
algorithm consist of the entire LTP segment except the AuthVal.
In particular, the header extensions field that may contain the
ciphersuite number and the KeyID field is part of the input.
The output bytes of the cryptographic operation are the payload of
the AuthVal field.
The following shows an example LTP-auth header, starting from and
including the Extensions field.
ext tag sdnv c-s k-id
+----+----+----+----+----+
|0x11|0x00|0x02|0x00|0x24|
+----+----+----+----+----+
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The Extensions field has the value 0x11 with the most significant and
least significant nibble value 1, indicating the presence of one
header and one trailer extension, respectively. The next octet is
the extension tag (0x00 for LTP-auth), followed by the Self-
Delimiting Numeric Value (SDNV) encoded length of the ensuing data: a
one-octet ciphersuite (0x00 meaning HMAC-SHA1-80) and the KeyID (in
this case with a short value of 0x24). The trailer extension (not
shown above) should contain the AuthVal.
2.2. A Cookie Mechanism
The use of cookies is a well-known way to make Denial of Service
(DoS) attacks harder to mount. We define the cookie extension for
use in environments where an LTP implementation is liable to such
attacks.
The cookie is placed in a header extension field, and has no related
trailer extension field. It has the extension tag value 0x01.
The cookie value can essentially be viewed as a sufficiently long
random number, where the length can be determined by the
implementation (longer cookies are harder to guess and therefore
better, though using more bandwidth). Note that cookie values can be
derived using lots of different schemes so long as they produce
random-looking and hard-to-predict values.
The first cookie inserted into a segment for this session is called
the initial cookie.
Note that cookies do not outlast an LTP session.
The basic mode of operation is that an LTP engine can include a
cookie in a segment at any time. After that time, all segments
corresponding to that LTP session MUST contain a good cookie value --
that is, all segments both to and from the engine MUST contain a good
cookie. Clearly, there will be some delay before the cookie is seen
in incoming segments -- implementations MUST determine an acceptable
delay for these cases, and MUST only accept segments without a cookie
until that time.
The cookie value can be extended at any time by catenating more
random bits. This allows both LTP engines to contribute to the
randomness of the cookie, where that is useful. It also allows a
node that considers the cookie value too short (say due to changing
circumstances) to add additional security. In this case, the
extended cookie value becomes the "to-be-checked-against" cookie
value for all future segments (modulo the communications delay as
above).
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RFC 5327 LTP - Extensions September 2008
It can happen that both sides emit segments containing an initial
cookie before their peer has a chance to see any cookie. In that
case, two cookie extension fields MUST be included in all segments
subsequently (once the traffic has caught up). That is, the sender
and recipient cookies are handled independently. In such cases, both
cookie values MUST be "good" at all relevant times (i.e., modulo the
delay). In this case, the peer's initial cookie MUST arrive before
the calculated delay for receipt of segments containing this engine's
cookie -- there is only a finite window during which a second cookie
can be inserted into the session.
A "good" cookie is therefore one that starts with the currently
stored cookie value, or else a new cookie where none has been seen in
that session so far. Once a cookie value is seen and treated as
"good" (e.g., an extended value), the previous value is no longer
"good".
Modulo the communications delay, segments with an incorrect or
missing cookie value MUST be silently discarded.
If a segment is to be retransmitted (e.g., as a result of a timer
expiring), then it needs to contain the correct cookie value at the
time of (re)transmission. Note that this may differ from what was
the correct cookie value at the time of the original transmission.
3. Security Considerations
The extensions specified above are generally intended to help thwart
DoS attacks. For environments where lower layers provide neither
integrity nor freshness, it makes sense to use both extensions
together. For example, in the case where a node extends an existing
cookie, the lack of origin authentication would allow a man in the
middle to lock out the session.
While there are currently some concerns about using the SHA-1
algorithm, these appear to only make it easier to find collisions.
In that case, the use of HMAC with SHA-1 can still be considered
safe. However, we have changed to use SHA-256 for the signature
ciphersuite.
4. IANA Considerations
IANA has created and now maintains registry for known LTP
ciphersuites (as defined in Section 2.1). The registry has been
populated using the initial values given in Sections 2.1 and 2.2
above. IANA may assign LTP Extension Tag values from the range
2..127 (decimal, inclusive) using the Specification Required rule
[GUIDE]. The specification concerned can be an RFC (whether
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RFC 5327 LTP - Extensions September 2008
Standards Track, Experimental, or Informational), or a specification
from any other standards development organization recognized by IANA
or with a liaison with the IESG, specifically including CCSDS
(http://www.ccsds.org/).
5. Acknowledgments
Many thanks to Tim Ray, Vint Cerf, Bob Durst, Kevin Fall, Adrian
Hooke, Keith Scott, Leigh Torgerson, Eric Travis, and Howie Weiss for
their thoughts on this protocol and its role in Delay-Tolerant
Networking architecture.
Part of the research described in this document was carried out at
the Jet Propulsion Laboratory, California Institute of Technology,
under a contract with the National Aeronautics and Space
Administration. This work was performed under DOD Contract DAA-B07-
00-CC201, DARPA AO H912; JPL Task Plan No. 80-5045, DARPA AO H870;
and NASA Contract NAS7-1407.
Thanks are also due to Shawn Ostermann, Hans Kruse, and Dovel Myers
at Ohio University for their suggestions and advice in making various
design decisions. This work was done when Manikantan Ramadas was a
graduate student at the EECS Dept., Ohio University, in the
Internetworking Research Group Laboratory.
Part of this work was carried out at Trinity College Dublin as part
of the Dev-SeNDT contract funded by Enterprise Ireland's technology
development programme.
6. References6.1. Normative References
[B97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[GUIDE] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[LTPSPEC] Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
Transmission Protocol - Specification", RFC 5326,
September 2008.
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RFC 5327 LTP - Extensions September 2008
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